Polymer mold-friendly design guidelines

G&H | GS Optics is recognized as one of the world’s leading manufacturers of custom designed polymer molding for optics. Every day we work with executives and program managers to solve complex optical challenges. From our location in Rochester, New York, USA, our team has been providing innovative solutions in precision polymer optics, since 1980, to small, medium and large size companies.

We can produce molded aspheric lenses and free-form lenses, and mirrors, as well as Fresnel and diffractive optics. We also have the in-house capability to provide custom designed diamond turned and injection molded prototypes, thin film, and reflective coatings, and integrated optical solutions for a broad range of consumer, medical, instrument LED lighting, and biomedical and analytical instrument marketplaces, as well as military and civilian night-vision and visible-range sighting industries.

Our in-house team consists of experts in single point diamond turning, thin film coating, mold fabrication, and optomechanical assembly, and we are ready to work with our clients’ technical teams to create high-performance, molded, optical element solutions for their program needs. G&H | GS Optics is DDTC registered and has a mature export compliance program in place.

Polymer Molding For Injection Molded Optics

Polymer molding requires successful plastic optics design requires careful consideration of the manufacturing processes involved. Transfer of the glass design to plastic by just replacing the material specification may lead to major problems down the road. Injection-molded optics should be designed with the manufacturing process in mind.

The following guidelines should help make an optical design mold friendly:

• Provide draft angles for all outside edges, mounting flanges, and holes to allow for part ejection from the mold without distortion or damage.
• Avoid undercuts, indentations or protrusions in a part that will prevent withdrawal from a one-piece mold. This will reduce tooling cost and cycle time.
• Identify appropriate gate area where a gate vestige will have the least effect on the part mounting or performance, preferably, at the heaviest cross-section of the part.
• Use generous radii at all corners.
• The overall part design should be as symmetrical as possible to optimize the melt flow in the mold.
• The ideal shape for molding is a nearly uniform wall thickness. Extreme variations in part thickness can cause uneven flow characteristics.
• Because flat surfaces have a tendency to sink as they cool in the mold, one should add a surface of power on both sides of the optic whenever possible.

One should keep in mind that molding cost is directly correlated with cycle time, which is, in turn, driven by part thickness and complexity. Optics with thicker cross sections not only require increased cycle times to mold, but also present a greater challenge in terms of maintaining higher surface figure accuracy. On the other hand, a thinner optic presents fewer shrinkage-compensation issues and shorter cycle times, translating into less costly parts.

Other issues to be considered for designing parts for injection molding are:

• Clear aperture specification should take into account that surface accuracy near the edges of the optical surface is difficult to control.
• Secondary operation or higher tool complexity may be required for tight gate vestige tolerance.
• Generous flanges help to prevent handling issues and can protect optical surfaces in transit.
• Packaging should be included in the scope of the project design.